1. Field of the Invention
The present invention relates to an actuator which is driven by a shape memory element.
2. Description of the Related Art
A shape memory element undergoes a phase transition due to a change in a temperature, and has a change of shape. An actuator in which, the shape change of the shape memory element is used, has superior characteristics of a small size and a light weight.
For instance, in Japanese Patent Application Laid-open Publication No. 1982-141704, a technology in which, the shape of a shape memory element is changed by supplying an electric power to the shape memory element, and a resistance value which changes with the shape change is detected, and by controlling an expansion and a contraction of the shape memory element, the actuator also functions as a sensor, has been disclosed.
Moreover, in Japanese Patent Application Laid-open Publication No. 2006-183564, an example of suppressing an excessive power supply to the shape memory element at the time of driving, by providing a limit condition to a control at the time of controlling upon by detecting the resistance value of the shape memory element has been introduced. According to this method, it is possible to achieve a highly reliable shape memory alloy actuator by suppressing an excessive heat to the shape memory alloy.
When the shape memory alloy is a wire material, a magnitude of the shape change of the shape memory alloy related to the drive is proportional to a length of the shape memory alloy. Consequently, for increasing a driving stroke, it is necessary to increase the length of the shape memory alloy.
For example, as in Japanese Patent Application Kokoku Hei 5-87677, the wire material of the shape memory alloy is inserted into a flexible tube having an insulating property. One end of the wire is fixed together with the flexible tube, and is let to be a fixed portion, and the other end of the wire is let to be a movable portion. By making such an arrangement, the movable portion performs its function even when the wire material of the shape memory alloy is bent together with the flexible tube. Therefore, in a unit in which, there is a tube site (portion) having a bending property, other than a driving portion, the shape memory alloy is incorporated in a tube position, and a long shape memory element is accommodated, it is possible to increase the driving stroke, which is an advantage.
In an actuator in which the shape memory alloy of a wire material is used, in a case of increasing the driving stroke, and controlling the drive of a movable body by controlling the expansion and the contraction of the shape memory alloy, a structure in which, the technology described in Japanese Patent Application Laid-open Publication No. 1982-141704 and Japanese Patent Application Kokoku Hei 5-87677 is let to be the basis, and the control is carried out by detecting the resistance value of the shape memory alloy, and the shape memory alloy is interpolated into the flexible tube is assumed.
Moreover, as in the technology described in Japanese Patent Application Laid-open Publication No. 2006-183564, providing the limit condition to the control for avoiding the excessive heating when the electric power is supplied at the time of controlling the expansion and the contraction of the shape memory alloy by detecting the resistance value is assumed Here, the resistance value of the shape memory alloy might change with the lapse of time, due to an effect of a change in an external environment, and a residual stress due to the usage. Consequently, for avoiding the excessive heating due to passing the electric supply, it is more desirable to reset the limit condition by actually letting the shape memory alloy to be contracted by heating at the beginning of the use every time, as it is possible to drive with the accurate limit condition every time, and the stability is improved.
When the control unit is provided with a step of setting the limit condition at the time of use every time as described above, in a flexible shape memory element actuator in which, a large driving stroke is secured, a frictional force added to the shape memory element by a magnitude of a bending angle becomes substantial. For this reason, for carrying out a step of setting a limit resistance value in a short time, it is necessary to increase an applied voltage in the limit resistance value setting step such that, the shape memory alloy undergoes a predetermined thermal expansion and contraction in a short time.
However, as it has been described in reference literature ‘Position control of SMA actuator in which hysteresis model is used’ (No. 640 by Kazuyuki Kodama, volume 65 (1992-12) of Journal of the Japan Society of Mechanical Engineers (Edition C)), when a voltage input is increased, or in other words, when a voltage gain is increased at the time of controlling the resistance value of the shape memory element to a predetermined target resistance value by a resistance feedback control for stopping a position of the movable body, there is a possibility that a vibration of the movable body becomes substantial when the resistance value of the shape memory element is close to the target resistance value.
Moreover, when the resistance value of the shape memory alloy does not attain the target resistance value at the time of moving the movable body by displacing the shape memory element up to a limit of a movable range, and stopping at a position of a mechanical restraint, even when the limit condition is set, in the resistance feed back control, an excess electric power equivalent to the amplitude of vibration is supplied to the shape memory element. Therefore, the shape memory element is heated excessively, and there is a possibility that a deterioration of performance of the shape memory element advances.